A mayenite-type compound is a 12CaO.7Al2O3 (hereinafter also referred to as “C 12A7”) crystal and a compound of the same type having a crystal structure equivalent to that of the C12A7 crystal, and has three-dimensionally linked voids (cages) having a diameter of about 0.4 nm. A framework constituting the cages carries a positive electric charge and forms 12 cages per unit lattice. One sixth of the cages are occupied by oxygen ions in order to satisfy an electrically neutral condition of the crystal, and these oxygen ions are particularly called “free oxygen ions” because they have properties chemically different from those of the other oxygen ions constituting the framework. Accordingly, the C12A7 crystal is also denoted as [Ca24Al28O64]4+.2O2− (Non-Patent Document 1).
Further, when the above-mentioned free oxygen ions are substituted by electrons, a mayenite-type compound having electrical conductivity is obtained. In Patent Document 1, it is described that when a C12A7 crystal is subjected to a reduction treatment by using alkali earth metal vapor, the free oxygen ions in the cages are substituted by electrons to be able to prepare a mayenite-type compound which is the C12A7 crystal and has electrical conductivity (hereinafter also referred to as a “C12A7 electride”).
Such a C12A7 electride has characteristics such as excellent sputtering resistance and secondary electron emission coefficient, so that the C12A7 electride has been considered to be preferably available, for example, as a material for a plasma display panel (hereinafter also referred to as a “PDP”).
However, in the C12A7 electride, the electrons in the cages rapidly diffuse and are immediately substituted by the oxygen ions. For this reason, the C12A7 electride is difficult to be stored in air for a long period of time. Further, heat treatment at the time when used in the PDP and the like decreases the electron concentration in the C12A7 electride, resulting in deterioration of characteristics such as the secondary electron emission coefficient.
Accordingly, there has been required a mayenite-type compound in which the deterioration of characteristics as described above is decreased even when heat treated at about 250 to 500° C., and which are excellent in thermal stability and oxidation resistance.
Then, Non-Patent Document 2 has proposed a method of replacing a part of free oxygen ions in a C12A7 crystal by electrons and hydride ions (H−), thereby improving oxidation resistance.